Please refer to FIG. 1 for a slot machine 1, which is one of the popular gaming machines. After a player stakes a bet, a plurality of reels 2 can be activated to spin. When the reels 2 stop, the alignments of symbols are formed to obtain scores for a bonus game. If a specific symbol is appeared or satisfied a preset condition, the player is awarded to play the bonus game. The bonus game has features of free spins, multiple scores and lock of appeared wild symbol. Hence the player has greater probability to obtain higher bonus game scores without extra expense and is excited by the expectation of hitting jackpot. This is one of the important features of the slot machine 1 to attract players.
However, under completely random setting, even in the bonus game with greater probability of winning, there is also possibility of failing to obtain alignments to hit jackpot and results in no wining or less scores awarded. The player could be easily discouraged or feel frustrated because of repetitive disappointments of loss or just gaining small awards amid the expectation of winning jackpot, and the motivation to continuously play the slot machine 1 is diminished. As a result, utilization of the slot machine 1 drops, and revenue of the slot machine 1 also decreases.